Synthesis and characterization of high performance polybenzoxazoles

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Abstract

High molecular weight poly(hydroxy amide)s and polybenzoxazoles
possessing a variety of backbone compositions have been
synthesized via low temperature polycondensation and a novel
catalytic cyclization reaction. Preliminary experiments involved
the synthesis of poly(hydroxy amide)s and polybenzoxazoles based
on a bisphenol-A aminophenol and isomeric acid chlorides. The
polymers exhibited Tg's around 280Â°C, limited solubilities and only
fair thermo-oxidative stabilities. In an effort to improve these
properties. fluorinated monomers were investigated. A number of
novel fluorinated monomers and commercial monomers were
incorporated into poly(hydroxy amide)s and polybenzoxazoles using
the above mentioned polymerization/cyclization techniques.
Enhanced solubilities, glass transition temperatures and thermooxidative
stabilities were realized. Difficulties encountered during
the polymerization of the fluorinated monomers prompted the study
of polymerization conditions. Results of these studies indicate that
the electronic environment about the bis(o-aminophenol)s and acid
chlorides dictates the ability of these monomers to participate in
an ester forming side reaction. Physical and spectroscopic evidence
was presented to support this conclusion. Investigations were also
conducted to determine optimum cyclization conditions. It was
shown that pyridine hydrochloride and p-toluenesulfonic acid
monohydrate were active cyclization catalysts when used in
conjunction with an azeotroping solvent, such as toluene or 0-
dichlorobenzene. In general, complete cyclization of the oxazole
rings could be achieved in as little as 6 hours at 175Â°C.
Unfortunately, generic cyclization conditions were not found.
Additionally, phenylethynyl terminated polybenzoxazoles were
successfully synthesized using the above techniques. Molecular
weight control and phenylethynyl incorporation were confirmed by
spectroscopic, thermal and mechanical analyses.